Conventionally, a biaxial hinge which couples a first casing of a notebook PC provided with a display portion and a second casing provided with a keyboard portion such that these casings can open and close relative to each other via a synchronous rotation mechanism is known from JP Laid-Open Patent Application No. 2016-1052. The synchronous rotation mechanism of the known biaxial hinge comprises a first gear and a second gear respectively attached to a first hinge shaft and a second hinge shaft, an intermediate gear provided between a first gear and a second gear; the synchronous rotation mechanism is structured such that it allows the first casing and the second casing to synchronously open and close, so that a biaxial hinge which allows for a satisfactory opening and closing operation and has a favorable efficiency and operability.
In the same manner, a triaxial hinge comprising a synchronous rotation mechanism between a first hinge shaft and a second hinge shaft is also disclosed by an application according to JP Patent Application No. 2016-96374 and a quadraxial hinge—by an application according to JP Patent Application No. 2015-50454.
On the other hand, though a liquid crystal using a glass plate was widely used as a display portion attached to a second casing, in recent years, a (flexible) display sheet of flexible material wherein a liquid crystal is sandwiched by resin sheets of flexible material, as is described in JP Laid-Open Patent Application No. H10-26754, and a (flexible) organic EL (organic luminescence) of flexible material, as is described in JP Laid-Open Patent Application No. 2009-187509, came into practical use, and large-sized, flexible display sheets are now acceptable to users also in terms of price. Therefore, not only the second casing but also the first casing of the notebook PC are entirely covered with the above-mentioned touch-panel type display sheet, so that a large-screen display becomes available without a keyboard on the first casing, while a touch-panel function also assures functionality of keyboard.
In this case, if the first casing and the second casing are coupled using the conventional hinges as described e.g. in the above-mentioned patent documents, that is JP Laid-Open Patent Application No. 2016-1052, JP Patent Applications Nos. 2016-96374 and 2015-50454, there are problems with flexible and touch operable display sheet of organic EL substrate, etc. being attached to the casings so as to cover the both sides of the hinges that, when they are bent in order to close the first casing and the second casing, they are bent too much so that they easily damage the display sheet of organic EL substrate, etc., and that, if they have rugged surfaces, a force unevenly applied to localized spot(s) on the display sheet during touch input easily causes damages to the display sheet.